This invention relates generally to gas turbine propulsion systems and more particularly to gas turbine propulsion systems that are convertible between two modes of operation. Rotorcrafts such as helicopters rely primarily on large rotating blades to produce both the lift necessary to stay aloft and the thrust necessary for propulsion. In order to produce both thrust and lift, the blades rotate in a plane generally parallel to the velocity vector of the craft, with blades advancing and retreating in the direction of flight of the craft. Due to low relative air speed or excessive angle of attack of the retreating blades, various flight instabilities and aerodynamic inefficiencies arise which limit the maximum, safe airspeed of a typical rotorcraft. In order to overcome the maximum velocity ceiling of rotorcrafts, various designs have incorporated secondary propulsion system to provide additional thrust. For example, rotorcrafts have incorporated rear propellers that rotate in a plane generally perpendicular to the velocity vector of the aircraft that produce only forward (or backward) thrust. Such secondary propulsion systems require input power that is typically siphoned off of the power supplied to the primary rotor blades.
Gas turbine propulsion systems produce a large amount of rotating shaft power available to both primary and secondary propulsion systems, and thus are popular choices for dual propulsion rotorcrafts such as helicopters. Typically, a turboshaft design is used wherein a gas generator is used to drive a gas generating turbine to compress inlet ambient air and sustain combustion, and a power turbine that drives a free shaft, which is then coupled to the rotor blades through a gearbox. A supplemental output from the free shaft can also be coupled to a secondary propulsion system, such as a propeller, to produce additional thrust. Typically, the supplemental output from the free shaft is mechanically coupled with the secondary propulsion system with a mechanical clutch actuation system. To engage the secondary propulsion system, clutch-type actuation mechanisms mechanically couple the secondary propulsion system with the primary propulsion system. Thus, not only does the secondary propulsion system directly reduce the amount of power available to the primary propulsion system, the mechanical clutch coupling limits free operation of the secondary propulsion system since the secondary propulsion system must rotate at speeds dictated by the free shaft. Thus, there is a need for a convertible propulsion system that provides greater flexibility in distributing power between primary and secondary propulsion systems.